Multifunctional Protein Molecular Weight Ladders

ABSTRACT

Multifunctional molecular weight protein ladders and methods of making thereof are disclosed herein that are useful for determining the molecular weight of a test protein and/or the relative mass or amount of the test protein in a protein separation assay, such as gel electrophoresis or western blotting. Also included are compounds of Formula I (e.g., mono acetylated MP-11 NHS ester) that may be used to label purified proteins of the protein ladder. The MP-11 label protein ladder can be detected on a blotting membrane by exposing the microperoxidase to a suitable substrate, such as a chromogenic substrate or a chemiluminescent substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional of U.S. Ser. No. 14/857,661filed Sep. 17, 2015 (Allowed); which claims priority to U.S. ProvisionalPatent Application No. 62/053,043 filed Sep. 19, 2014; the fulldisclosures which are hereby incorporated by reference in their entiretyfor all purposes.

BACKGROUND OF THE INVENTION

A protein molecular weight marker or protein ladder can be used toidentify the approximate size (molecular weight) or mass (amount) of aprotein resolved by gel electrophoresis. As is recognized by thoseskilled in the art, the migration rate of a protein through a gel isinversely proportional to the protein's molecular weight.

There is a need in the field for protein ladders that can be visibly,fluorescently and chemiluminescently detected either simultaneously orsequentially. Additionally, simpler methods are needed forchemiluminescent systems that eliminate the need for the inclusion ofantibodies with a catalytic function label to develop achemiluminescence system.

Moreover, there is a need for improved methods of labeling a proteinmixture, wherein the proteins are conjugated to multiple chemicalmoieties. The present invention satisfies these needs and provides otheradvantages as well.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a compound of FormulaI:

wherein R¹, R², R³, and R⁴ are each members independently selected fromthe group consisting of hydrogen, an amine protecting group or L,wherein L is a linking group.

In a second embodiment, the present invention provides a proteinstandard comprising a plurality of purified proteins having differingmolecular weight; and at least a first portion of the purifiedprotein(s) is covalently labeled using a mono microperoxidase compoundof Formula I (e.g., N-succinimidyl (NHS) ester at R⁴). A “first portion”of the purified proteins can be one or more proteins.

In some aspects, the protein standard includes at least 5 differentpurified proteins. In other aspects, the protein standard includes atleast 10 different purified proteins (1, 2, 3, 4, 5, 6, 7, 8, 9 10 ormore). In some instances, the range of molecular weights of the proteinstandard is about 5 kDa to about 280 kDa. In other instances, the rangeof molecular weights of the protein standard is about 8 kDa to about 260kDa. In some aspects, the protein ladder includes purified proteins orsubstantially purified proteins that are about 250 kDa, about 150 kDa,about 100 kDa, about 75 kDa, about 50 kDa, about 37 kDa, about 25 kDa,about 20 kDa, about 15 kDa, and about 10 kDa. In other aspects, theprotein ladder includes purified proteins or substantially purifiedproteins that are about 250 kDa, about 130 kDa, about 100 kDa, about 70kDa, about 55 kDa, about 35 kDa, about 25 kDa, about 15 kDa, and about10 kDa. In some aspects, the protein ladder includes purified proteinsor substantially purified proteins that are about 260 kDa, about 140kDa, about 100 kDa, about 70 kDa, about 50 kDa, about 40 kDa, about 35kDa, about 25 kDa, about 15 kDa, and about 10 kDa. In other aspects, theprotein ladder includes purified proteins or substantially purifiedproteins that are about 260 kDa, about 160 kDa, about 90 kDa, about 70kDa, about 50 kDa, about 38 kDa, about 30 kDa, about 25 kDa, about 15kDa, and about 8 kDa. In another aspect, the protein ladder includespurified proteins or substantially purified proteins that are about 260kDa, about 160 kDa, about 90 kDa, about 50 kDa, about 30 kDa, about 15kDa, and about 8 kDa. In yet another aspect, the protein ladder includespurified proteins or substantially purified proteins that are about 260kDa, about 125 kDa, about 70 kDa, about 38 kDa, about 25 kDa, and about8 kDa.

In some aspects, at least 2 of the purified proteins of the proteinstandard are covalently labeled using a mono microperoxidase of FormulaI. In other aspects, at least 3 of the purified proteins of the proteinstandard are covalently labeled using a mono microperoxidase of FormulaI. In another aspect, at least 4 of the purified proteins of the proteinstandard are covalently labeled using a mono microperoxidase of FormulaI. In some aspects, at least 5 of the purified proteins of the proteinstandard are covalently labeled using a mono microperoxidase of FormulaI. In yet other aspects, at least 10 of the purified proteins of theprotein standard are covalently labeled using a mono microperoxidase ofFormula I.

In some aspects, at least a second portion of the purified proteins ofthe protein standard are labeled with an infrared (IR) fluorescent dyeor porphyrin dye. In some instances, the IR fluorescent dye is a cyaninedye or a phthalocyanine dye. The cyanine dye may be IRDye® 800CW, IRDye750, IRDye 680RD or IRDye 680LT (LI-COR, Lincoln, Nebr.). Thephthalocyanine dye can be IRDye® 700DX (LI-COR, Lincoln, Nebr.). Thoseof skill in the art will know of other cyanine or phthalocyanine dyesuseful for the present invention.

In some aspects, the first portion and the second portion of thepurified proteins of the protein standard are the same purifiedproteins. In other aspects, the first portion and the second portion ofthe purified proteins of the protein standard are different purifiedprotein(s).

In one aspect, the first portion of purified proteins of the proteinstandard is one or more proteins. The second portion of the purifiedproteins of the protein standard is one or more proteins. The one ormore proteins of the first and second portion can be the same protein(s)or different protein(s).

In some aspects, at least a third portion of the purified proteins ofthe protein standard is labeled with a visible dye. In one aspect, thethird portion of the purified proteins of the protein standard is one ormore proteins. The one or more proteins of the first and second portioncan be the same protein(s) or different protein(s) compared to the thirdportion of the purified proteins of the protein standard.

In a third embodiment, the present invention provides a kit forpreparing a protein standard described herein. The kit includes aplurality of purified proteins of differing molecular weight, a monomicroperoxidase NHS ester (or other functional labeling group), and alabeling buffer. In some aspects, a mono microperoxidase NHS ester is amono MP-11 of Formula Ia:

In some aspects, Formula Ia is a mixture of compounds wherein R¹, R²,R³, and R⁴ are each members independently selected from hydrogen, an NHSester and an acetyl group such as R¹ is acetyl, R² is hydrogen, R³ ishydrogen and R⁴ is an NHS ester (e.g., with 6 CH₂ groups with an amidelinkage to the core). Alternatively, R¹ is an NHS ester and R³ is anacetyl group. There can be a mixture of isomers such as between 1% to99% of each of the isomers.

In some aspects, the labeling buffer is an amine-free buffer. In someaspects, the kit also includes a storage buffer. In preferred aspects,the storage buffer is substantially free of DTT, EDTA and NaN₃. Incertain aspects, the kits are substantially free of antibodiespossessing catalytic activity.

In a fourth embodiment, the present invention provides a method forpreparing a multifunctional protein standard comprising a plurality ofpurified proteins labeled with at least two chemical moieties selectedfrom the group consisting of a visible dye moiety, a catalytic moiety, afluorescent dye moiety, and a combination thereof. The method includes(a) determining the optimum number of molar equivalents of the at leasttwo chemical moieties based on the average molecular weight of theplurality of purified proteins when labeled individually; (b) preparingthe plurality of purified proteins buffers amenable to the conjugationchemistries of at least two chemical moieties; and (c) incubating the atleast two chemical moieties and the plurality of purified proteins underconditions such that the at least two chemical moieties covalentlyattach to the proteins to produce the multifunctional protein standard.In some aspects, the method also includes purifying the multifunctionalprotein standard. In some instances, the step of purifying themultifunctional protein standard comprises size exclusionchromatography, dialysis or both.

In some aspects, the step of incubating the at least two chemicalmoieties and the plurality of purified proteins comprises simultaneouslyconjugating the chemical moieties to the proteins. In other aspects,incubating the at least two chemical moieties and the plurality ofpurified proteins comprises sequentially conjugating the chemicalmoieties, wherein a chemical moiety with an NHS ester is conjugated tothe proteins after a chemical moiety without an NHS ester. In yet otheraspects, incubating the at least two chemical moieties and the pluralityof purified proteins comprises sequentially conjugating the chemicalmoieties, wherein the order of conjugating is from the least to the mostmolar equivalents needed for each chemical moiety.

In some aspects, at least two purified proteins of the multifunctionalprotein standard are labeled with a catalytic moiety and a visible dyemoiety using the method described herein.

In other aspects, at least two purified proteins of the multifunctionalprotein standard are labeled with a catalytic moiety and a fluorescentdye moiety.

In preferred aspects, at least two purified proteins of themultifunctional protein standard are labeled with (i) a catalyticmoiety, (ii) a visible dye moiety, (iii) a fluorescent dye moiety or anysingle or combination of the three, including (i) and (ii); (i) and(iii); (ii) and (iii); or (i), (ii), and (iii).

In some aspects, the step of incubating the at least two chemicalmoieties and the plurality of purified proteins includes (i) conjugatingthe visible dye moiety to the proteins, (ii) conjugating the fluorescentdye to the labeled proteins, and/or (iii) conjugating the catalyticmoiety to the fluorescently labeled proteins. In some instances, thecatalytic moiety is a mono microperoxidase NHS ester. In some aspects,the microperoxidase is a compound of Formula Ia:

In some aspects, the fluorescent dye moiety is an infrared fluorescentdye or porphyrin dye. In some instances, the infrared fluorescent dye isa cyanine dye or a phthalocyanine dye. In some aspects, the cyanine dyeis IRDye® 800CW, IRDye 750, IRDye 680RD, IRDye 650 or IRDye 680LT. Insome aspects, the phthalocyanine dye is IRDye® 700DX.

In another aspect, the present invention provides a multifunctionalprotein standard prepared by any of the methods described herein.

In yet another embodiment, the present invention provides a method forperforming a Western blot analysis, the method comprising:

-   -   (a) loading a protein standard comprising a plurality of        purified proteins of differing molecular weight, wherein at        least a first portion of the purified proteins is covalently        labeled with a mono microperoxidase (MP) N-hydroxy succinimidyl        (NHS) ester;    -   (b) electrophoresing the plurality of proteins to form a        separated protein mixture;    -   (c) transferring the separated protein mixture to a membrane;    -   (d) adding a chemiluminescent substrate to form a chemi-signal;        and    -   (e) capturing the chemi-signal with a film or an imager.

In some aspects, the membrane is nitrocellulose or PVDF. In someaspects, an unknown protein is loaded with the protein standard.

Other objects, features, and advantages of the present invention will beapparent to one of skill in the art from the following detaileddescription and figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a two-step method of synthesizing mono N-acetylatedMP-11 NHS ester.

FIGS. 2A-2C show an exemplary embodiment of a multifunctional markerdescribed herein. The protein marker ladder contains pre-stainedproteins with visible dyes (FIG. 2A), chemiluminescent signal (FIG. 2B)and IR fluorescent moieties (FIG. 2C).

FIGS. 3A and 3B show the stability of an exemplary embodiment of amultifunctional marker described herein. The marker ladder stored inDTT, NaN₃ and EDTA-free buffer retains its peroxidase-like activity.

FIGS. 4A and 4B show the stability of two multifunctional proteinladders labeled with mono MP-11 ester and stored at −80° C., −20° C. and4° C. FIG. 4A shows a labeled, pre-stained protein ladder with MP-11that was stored for 6 months. FIG. 4B shows a MP-11 labeled, pre-stainedand IR dye conjugated protein ladder that was stored for 5 months at−80° C., −20° C. and 4° C.

FIG. 5 shows representative visible (left) and chemiluminescent (right)images of a chemiluminescent protein ladder resolved on a 4-12% Bis-TrisGel and transferred to nitrocellulose via a wet tank transfer.

DETAILED DESCRIPTION OF THE INVENTION I. Introduction

The present invention provides a multifunctional protein molecularweight ladder that is useful for visualization, production ofchemiluminescence and infrared fluorescence in for example, a westernblotting analysis. The invention provides methods that demonstrate theversatility of using the ladder in both chemiluminescent and fluorescentwestern blotting analysis. For instance, the molecular weight and themass of a target protein can be estimated or determined using theprotein standard described herein having at least one chemiluminescencereporter and at least one infrared fluorescence reporter (e.g., IRDye®fluorescent dye). Also provided herein is a method of making forexample, an amino reactive mono microperoxidase that is useful forsimultaneously labeling a plurality of proteins, such as pre-stainedproteins of a molecular weight ladder. The present invention is alsodirected to labeling either subsequently or simultaneously such acatalytic protein ladder with one or more IR dyes such as near IR dyes.The invention is related to formulating a multifunctional proteinmolecular weight ladder that possesses up to three signal reporters in abuffer system that preserves (i) peroxidase-like activity, as well as(ii) visible and (iii) fluorescence properties of the ladder.

II. Definitions

The terms “a,” “an,” or “the” as used herein not only include aspectswith one member, but also include aspects with more than one member. Forinstance, the singular forms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to “a cell” includes a plurality of such cells andreference to “the agent” includes reference to one or more agents knownto those skilled in the art, and so forth.

The terms “about” and “approximately” shall generally mean an acceptabledegree of error for the quantity measured given the nature or precisionof the measurements. Typical, exemplary degrees of error are within 20percent (%), preferably within 10%, and more preferably within 5% of agiven value or range of values. Alternatively, and particularly inbiological systems, the terms “about” and “approximately” may meanvalues that are within an order of magnitude, preferably within 5-foldand more preferably within 2-fold of a given value. Numerical quantitiesgiven herein are approximate unless stated otherwise, meaning that theterm “about” or “approximately” can be inferred when not expresslystated.

The term “protein standard,” “protein ladder,” “molecular weight proteinmarker,” “molecular weight marker,” “protein molecular weight marker”refers to a plurality of purified proteins such as 1, 2, 3, 4, 5, 6, 7,8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or more proteins thatcan separated according to molecular weight by protein electrophoresisand are useful for molecular weight estimation or determination.

The term “plurality of purified proteins” includes a quantity of atleast two different full-length proteins or polypeptides, proteinfragments (e.g., peptides), denatured proteins or proteins in theirnative state that have been purified or substantially purified, e.g., 2,3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or more differentfull-length proteins or polypeptides, protein fragments, denaturedproteins or proteins in their native state that have been purified. Theplurality or mixture of purified proteins can be a mixture offull-length proteins or polypeptides, a mixture of protein fragments, amixture of denatured proteins, a mixture of proteins in their nativestate, or any combination thereof.

The term “mono microperoxidase N-hydroxy succinimidyl ester,” “monomicroperoxidase NHS ester” or “microperoxidase NHS ester” refers to aheme containing peptide portion of cytochrome C that retains peroxidaseactivity and having a N-hydroxy succinimidyl (NETS) ester reactivegroup. Non-limiting examples of microperoxidases that are useful for theinvention include MP-17 (having 17 amino acids), MP-9 (a nonapeptide),MP-8 (an octapeptide), and MP-6 (a hexapeptide). Any peptide derivedfrom a peroxidase protein by proteolysis or synthesis and having a NETSester group or other functional linking group can be used if the peptidepossesses enzymatic activity or peroxidase activity.

The term “conjugating,” “coupling” or “labeling” refers to linking of atleast one chemical moiety to a protein by means of a suitablecrosslinker capable of covalently binding the moiety to the protein.

The term “linking group” refers to a moiety on the compound that iscapable of chemically reacting with a functional group on a differentmaterial (e.g., biomolecule) to form a linkage, such as a covalentlinkage. See, e.g., R. Haughland, The Handbook—A Guide to FluorescentProbes and Labeling Technologies, 9^(th) Edition, Molecular Probes, Inc.(1992). Typically, the linking group is an electrophile or nucleophilethat can form a covalent linkage through exposure to the correspondingfunctional group that is a nucleophile or electrophile, respectively.Alternatively, the linking group is a photoactivatable group, andbecomes chemically reactive only after illumination with light of anappropriate wavelength. Typically, the conjugation reaction between thedye bearing the linking group and the material to be conjugated with thedye results in one or more atoms of the linking group being incorporatedinto a new linkage attaching the dye to the conjugated material.

The term “infrared fluorescent dye,” “IR fluorescent dye” or “IR dye” or“NIR dye” refers to a dye having an absorption and emission wavelengthsin the near-infrared spectrum of about 600-1000 nm. An infraredfluorescent dye can be detected using a near-infrared (NIR) fluorescenceimaging system.

The term “cyanine dye” refers to a compound having two substituted orunsubstituted nitrogen-containing heterocyclic rings joined by anunsaturated bridge, such as a polymethine chain. Non-limiting examplesof a cyanine dye, such as IRDye® 800CW are described in, e.g., U.S. Pat.Nos. 6,995,274; 7,504,089; 7,597,878; 8,227,621; and 8,303,936; thedisclosures of which are herein incorporated by reference in theirentirety for all purposes.

The term “phthalocyanine dye” refers to a silicon phthalocyanine dyethat are useful for conjugating to a biomolecule, such as a protein.Non-limiting examples of a phthalocyanine dye, such as JRDye® 700DX aredescribed in, e.g., U.S. Pat. No. 7,005,518, the disclosure of which isherein incorporated by reference in its entirety for all purposes.

The term “pre-stained protein standard” or “pre-stained protein ladder”refers to protein standards composed of proteins that are stained toallow for monitoring of the proteins during electrophoresis.

The term “chemical moiety” refers to a compound, chemical group,functional group or composition.

The term “visible dye moiety” or “visible dye” refers to a compound,chemical group, functional group, or composition that is visible to orcan be detected by the unaided human eye.

The term “fluorescent dye moiety” or “fluorophore” refers that isinherently fluorescent. Fluorophores may contain substituents that alterthe solubility, spectral properties or physical properties of thefluorophore. Numerous fluorophores are known to those skilled in the artand include, but are not limited to, coumarin, cyanine, benzofuran, aquinoline, a quinazolinone, an indole, a furan, a benzazole, aborapolyazaindacene and xanthenes including fluoroscein, rhodamine andrhodol as well as other fluorophores described in R. Haughland, TheHandbook—A Guide to Fluorescent Probes and Labeling Technologies, 9^(th)Edition, Molecular Probes, Inc. (1992).

The term “chemiluminescent moiety” or “chemiluminescent reagent” referto a chemical moiety that can react with a substrate derived fromluminol or an isomer thereof, including, but not limited to, luminal(3-aminophthalhydrazide) isoluminol (4-aminophthalhydrazide), ECL(Amersham), Clarity™ Western ECL substrate (Bio-Rad, Hercules, Calif.),SuperSignal (Thermo Fisher Scientific, Rockford, Ill.), or DuoLuX™(Vector Laboratories, Burlingame, Calif.).

The term “optimum number of molar equivalents” refers to the number ofindividual chemical moieties which provide optimum individualfunctionalities on a protein. For example, the optimum molar equivalentsof a visible dye produces a protein which has sufficient color to beseen by the unaided eye. The optimum molar equivalents of a fluorescentdye produces a protein that has sufficient fluorescent signal to beimaged on instruments such as LI-COR Oddysey Fc, Clx or Classic withoutsaturating the detector or coalescing protein bands of differentmolecular weights. The optimum molar equivalents of a catalyst produce aprotein that has sufficient chemiluminescent signal when incubated withan appropriate substrate and imaged on an instrument such as LI-COROdyssey Fc or C-DiGit imager without saturating the detector, or withoutcoalescing protein bands of different molecular weights. Optimal molarequivalents produce proteins which are sharp and well resolved whenseparated by SDS-PAGE.

III. Detailed Descriptions of Embodiments

A. Microperoxidase

Microperoxidases (MPs) are typically fragments of cytochrome C producedby enzymatic cleavage of the protein that results in segments of thecytochrome C amino acid chain with its heme group covalently attachedvia thioether bonds to the peptide. These small molecules have amolecular weight of less than 5,000 g/mol (5 kDa). Microperoxidases arecatalytic and have peroxidative or peroxidase activity. Microperoxidasessuitable for the present invention are commercially available. Forexample, MP-17, MP-11, MP-9, MP-8 and MP-6 are available from, e.g.,Sigma-Aldrich Chemical Company, St. Louis, Mo. Combinations ofmicroperoxidases are also useful.

Microperoxidases can be conjugated to one or more proteins in a proteinladder to produce a protein molecular weight ladder with catalyticactivity. In fact, the catalytic activity is retained underelectrophoresis and western blotting. The microperoxidase functionalizedprotein ladder can catalyze luminol-based chemical reactions to generatediscrete chemiluminescent bands on western blot membranes. If thereagent is designed to be used with a peroxidase, it can be used with amicroperoxidase.

Protein ladders without catalytic activity can be stored in a storagebuffer containing Tris, EDTA, optionally SDS, NaN₃, glycerol and DTT.Prior to synthesizing a catalytic protein ladder, the storage buffer canbe removed using standard methods, such as, but not limited to,utilizing a size exclusion column, e.g., spin column, and optionally,buffer exchange to a buffer compatible with the labeling reaction andalso retain protein solubility by optionally adding SDS. In someaspects, the buffer comprises about 25 mM to about 100 mM sodiumphosphate at about pH 8.5 with optionally 0.1%-1% SDS. In certaininstances, SDS is included. In other instances, SDS is not included.

In one embodiment, the present invention provides a compound of FormulaI:

wherein R¹, R², R³, and R⁴ are each members independently selected fromthe group consisting of hydrogen, an amine protecting group or L,wherein L is a linking group.

Suitable amine protecting groups include, but are not limited to, anacetyl (Ac), a benzoyl (Bz), a benzyl (Bn) group, tert-butyloxycarbonyl(BOC) group, a carbamate group, a carbobenzyloxy (Cbz) group,3,4-dimethoxybenzyl (DMPM), 9-fluorenylmethyloxycarbonyl (FMOC) group,p-methoxybenzyl carbonyl (Moz) group, p-methoxybenzyl (PMB),p-methoxyphenyl (PMP) group, or Tosyl (Ts) group. In one aspect, R¹ isCH₃C(O) and R² is hydrogen.

Suitable linking groups L in Formula I are groups that can covalentlyreact with an amine, hydroxyl, or sulfhydryl for bioconjugation,crosslinking, labeling and/or immobilization. These groups can be, forexample, an NHS ester, a sulfo-NHS ester, N-maleimide, N-succinimide, orhydrazide moieties.

For example, N-Hydroxysuccinimide (NETS) is an organic compound with theformula C₄H₅NO₃. NETS is used as an activating reagent for carboxylicacids. For example, activated acids (basically esters with a goodleaving group) can react with amines on proteins to form amides. Informula I, any of R¹-R⁴ can be an NETS ester, wherein the NETS ester islinked to the microperoxidase via a methylene(s) group and an amidebond. The number of methylene(s) can be 1-20, such as 1, 2, 3, 4, 5, 6,7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or more (see, FIG.1, step 2, with DSS (disuccinimidyl suberate)). The methylene groups canbe optionally interrupted with heteroatoms. A sulfo-NHS can be used toincrease water solubility.

An N-maleimide is a chemical compound with the formula H₂C₂(CO)₂NH,wherein the hydrogen on the nitrogen is replace and linked to themicroperoxidase. The maleimide can react with an amine or sulfhydrylgroup on a protein. Other groups include, but are not limited to,isothiocyanates, isocyanates, acyl azides, sulfonyl chlorides,aldehydes, glyoxals, epoxides, oxiranes, carbonates, aryl halides,imidoesters, carbodiimides, anhydrides, and fluorophenyl esters. Most ofthese conjugate to amines on proteins by either an acylation or analkylation reaction.

In certain aspects, the compound of Formula I can be conjugated toproteins using conjugation chemistry well known in the art. For example,an activated ester (an NETS ester) can react with a primary amine tomake a stable amide bond. A maleimide on Formula I and a thiol group ona protein can react together to make a thioether. Alkyl halides onFormula I react with amines and thiols to make alkylamines andthioethers, respectively. Any derivative providing a reactive moietythat can be conjugated to a protein can be utilized herein. As is knownin the art, moieties comprising a free amino group, a free carboxylicacid group, or a free sulfhydryl group provide useful reactive groupsfor conjugation. For example, a free amino group on the MP can beconjugated to a protein via glutaraldehyde cross-linking, or viacarbodiimide cross-linking to available carboxy moieties on the protein.Also, a protein with a free sulfhydryl group can be conjugated to a MPvia maleimide activation, e.g., usingsulfosuccinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate(Sulfo-SMCC), then linkage to the sulfhydryl group.

In certain instances, “L” is “L^(c)” which is a covalent link between anMP such as MP-11 and a protein. In certain aspects, L^(c) comprises acovalent bond from column “C” in Table 1 between an MP and a protein. Inother words, L^(c) is the resulting bond between an MP having a reactivefunctional group and a protein. L^(c) can be for example, an amide, athioether, or an alkylamine.

Methods for individually conjugating a protein are known to thoseskilled in the art, such as methods using reagents containingN-maleimide, N-succinimide, or hydrazide moieties that selectively reactwith sulfhydryl groups, amino groups or aldehyde groups, respectively.N-succinimide modified MP-11 conjugation can be performed at any of thefour carboxyl or two amino groups of MP-11. However, crosslinkingbetween MP-11 and the protein ladder may occur such that the discretebands of the ladder do not appear on western blotting. Thus, blockingthe carboxyl and/or amino groups is sometimes useful to synthesize aMP-11 having NHS ester functionality.

Selected examples of reactive functionalities useful for attaching acompound of Formula I to proteins are shown in Table 1, wherein the bondresults from the reaction of a compound of Formula I with a protein.Column A of Table 1 is a list of the reactive functionalities, which canbe on the compound of Formula I or the protein. Column B is a list ofthe complementary reactive groups (preferably, a carboxyl, hydroxyl,thiol, or amino functionality), which can be on the protein or thecompound of Formula I, and which react with the indicated functionalityof Column A to form the bond of Column C. Those of skill in the art willknow of other bonds suitable for use in the present invention.

TABLE 1 Exemplary Bonds for Linking Groups B Complementary A GroupReactive Functionality (protein or C (Compound of Formula I Compound ofResulting or protein) Formula I) Linking Group activated esters*amines/anilines amides acrylamides thiols thioethers acyl azides**amines/anilines amides acyl halides amines/anilines amides acyl halidesalcohols/phenols esters acyl nitriles alcohols/phenols esters acylnitriles amines/anilines amides aldehydes amines/anilines iminesaldehydes or ketones hydrazines hydrazones aldehydes or ketoneshydroxylamines oximes alkyl halides amines/anilines alkyl amines alkylhalides carboxylic acids esters alkyl halides thiols thioethers alkylhalides alcohols/phenols ethers anhydrides alcohols/phenols estersanhydrides amines/anilines amides/imides aryl halides thiols thiophenolsaryl halides amines aryl amines azides alkynes 1,2,3-triazolesaziridines thiols thioethers boronates glycols boronate esters activatedcarboxylic amines/anilines amides acids activated carboxylic alcoholsesters acids activated carboxylic hydrazines hydrazides acidscarbodiimides carboxylic acids N-acylureas or anhydrides diazoalkanescarboxylic acids esters epoxides thiols (amines) thioethers (alkylamines) epoxides carboxylic acids esters haloacetamides thiolsthioethers haloplatinate amino platinum complex haloplatinateheterocycle platinum complex halotriazines amines/anilinesaminotriazines halotriazines alcohols/phenols triazinyl ethers imidoesters amines/anilines amidines isocyanates amines/anilines ureasisocyanates alcohols/phenols urethanes isothiocyanates amines/anilinesthioureas maleimides thiols thioethers phosphoramidites alcoholsphosphite esters silyl halides alcohols silyl ethers sulfonate estersamines/anilines alkyl amines sulfonyl halides amines/anilinessulfonamides *Activated esters, as understood in the art, generally havethe formula —C(O)OM, where —OM is a leaving group (e.g. succinimidyloxy(—OC₄H₄NO₂), sulfosuccinimidyloxy (—OC₄H₃NO₂SO₃H), -1-oxybenzotriazolyl(—OC₆H₄N₃); 4-sulfo-2,3,5,6-tetrafluorophenyl; or an aryloxy group oraryloxy substituted one or more times by electron withdrawingsubstituents such as nitro, fluoro, chloro, cyano, or trifluoromethyl,or combinations thereof, used to form activated aryl esters; or —C(O)OMis a carboxylic acid activated by a carbodiimide to form an anhydride ormixed anhydride —C(O)OC(O)R^(a) or —C(O)OC(NR^(a))NHR^(b), wherein R^(a)and R^(b) are members independently selected from the group consistingof C₁-C₆ alkyl, C₁-C₆ perfluoroalkyl, C₁-C₆ alkoxy, cyclohexyl,3-dimethylaminopropyl, or N-morpholinoethyl). **Acyl azides can alsorearrange to isocyanates.

In one aspect, the L in Formula I is an N-succinimide ester with between2 and 12 methylene groups (such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12CH₂ groups) with an amide linkage to a MP (e.g. MP-11). In otherinstances, L is a member selected from the group of a direct link, or acovalent linkage, wherein said covalent linkage is linear or branched,cyclic or heterocyclic, saturated or unsaturated, having 1-60 atomsselected from the group consisting of C, N, P, O, and S, wherein L canhave additional hydrogen atoms to fill valences, wherein said linkagecontains any combination of ether, thioether, amine, ester, carbamate,urea, thiourea, oxy or amide bonds; or single, double, triple oraromatic carbon-carbon bonds; or phosphorus-oxygen, phosphorus-sulfur,nitrogen-nitrogen, nitrogen-oxygen, or nitrogen-platinum bonds; oraromatic or heteroaromatic bonds.

In other instances, L is a member selected from the group consisting ofa PEG, a block copolymer of PEG-polyurethane and a PEG-polypropylene.

In other instances, L is a member selected from the group consisting ofa polysaccharide, a polypeptide, an oligosaccharide, a polymer, aco-polymer and an oligonucleotide.

In other instances, L is of the formula:

—X¹—Y¹—X²—

wherein:

X¹ is a member selected from the group consisting of a bivalent radical,a direct link, oxygen, an optionally substituted nitrogen and sulfur;

Y¹ is a member selected from the group consisting of a direct link andC₁-C₁₀ alkylene optionally interrupted by a heteroatom; and

X² is a member selected from the group consisting of a bivalent radical,a direct link, oxygen, an optionally substituted nitrogen and sulfur.

L^(c) is the resulting bond between L and a protein. Typically, Lcontains a reactive group which reacts with an amino, a sulfhydryl, ahydroxyl, or a carboxyl group of a protein and forms L^(c). (See, Table1.)

In one aspect, Formula I is Formula Ia:

Microperoxidase-11 (e.g., MP-11) is prone to aggregation in solution,via axial ligation and intermolecular interactions (Kadnikova andKostić, J Org Chem, 2003, 68(7):2600-8). The present invention is based,in part, on the surprising discovery of a two-step synthesis reactionfor the production of a mono MP-11 NHS ester (Formula I). First, toeliminate MP-11 aggregation, the first primary amino group of themicroperoxidase can be blocked using a reagent with an amino reactivegroup including, but not limited to, isothiocyanates, isocyanates,acyl-azides or a derivative thereof. The blocking reaction can beperformed in an aqueous solution with any water-soluble mono NHS ester.In some aspects, the blocking reagent is sulfosuccinimidyl acetate(sulfo-NHS-acetate) in an aqueous solution, such as a phosphate bufferat about pH 8.5. The reaction when performed with low levels of MP-11and stepwise added sulfosuccinimidyl acetate into reaction can alsoimprove the yield of Mono-acetyl MP-11. A less than or equal to 1 mg/mlof MP-11 was used for reducing MP-11 aggregation and di-acetyl MP-11 aswell. After the blocking reaction, the resulting Mono N-acetylated MP-11can be purified using an acid, such as an acid having a pH of about pH2-5.

MP-11 is insoluble (e.g., completely insoluble) in organic solvents.Yet, an amino blocked MP-11 (e.g., an N-acetylated MP-11) has increasedsolubility in organic solvents and can be reacted with a water-insolublecrosslinker, such as a homobifunctional NHS ester.

In the second step of the synthesis reaction, standard NHS chemistry canbe used to label the modified MP-11. In some aspects, the acetylatedMP-11 is reacted in an organic solvent, e.g., anhydrous dimethylsulfoxide (DMSO) to a water-insoluble homobifunctional NHS ester, e.g.,disuccinimidyl suberate (DSS). In other aspects, the acetylated MP-11 isdissolved in pyridine and admixed with the crosslinker solution that cancontain N,N-diisopropylethylamine (DIPEA).

The NHS ester (or succinimidyl ester) of microperoxidase MP-11 can beused to conjugate MP-11 to the primary amines (R—NH₂) of purifiedproteins. The conjugation forms an amide bond between MP-11 and theprotein. It can react with non-protonated aliphatic amine groups,including the amine terminus and the ε-amino group of lysines of theprotein.

B. Multifunctional Protein Ladders

The compositions and methods of the present invention provide a proteinmolecular weight ladder comprising multiple purified proteins that arelabeled with at least one microperoxide (e.g., MP-11) moiety and eitherat least one visible dye, at least one fluorescent dye, or both dyes. Insome aspects, at least one protein is labeled with both the MP-11 moietyand a visible dye. For instance, the protein can be selectively labeledwith a visible dye on a first amino acid and labeled with MP-11 at adifferent amino acid. In other aspects, at least one protein is labeledwith both the MP-11 moiety and a fluorescent dye. For instance, theprotein can be selectively labeled with a fluorescent dye on a firstamino acid and labeled with MP-11 at a different amino acid. In yetother aspects, at least one protein is labeled with (i) the MP-11moiety, (ii) a visible dye, and (iii) a fluorescent dye. For instance,the protein can be selectively labeled with a visible dye on a firstamino acid, labeled with MP-11 at a second amino acid, and labeled witha fluorescent dye at a third amino acid.

In some aspects, the molecular weight standard includes 2, 3, 4, 5, 6,7, 8, 9, 10, 11, 12, 13, 14, 15 or more different, labeled proteins inwhich 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or more proteinsare selectively labeled with at least one MP-11 moiety on a first aminoacid and selectively labeled with at least one visible dye on a secondamino acid. In other aspects, the molecular weight standard includes 2,3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or more different, labeledproteins in which 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 ormore proteins are selectively labeled with at least one MP-11 moiety ona first amino acid and selectively labeled with at least one fluorescentdye on a second amino acid. In yet other aspects, the molecular weightstandard includes 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or moredifferent, labeled proteins in which 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,12, 13, 14, 15 or more proteins are selectively labeled with at leastone MP-11 moiety on a first amino acid, selectively labeled with atleast one visible dye on a second amino acid, and selectively labeledwith at least one fluorescent dye on a third amino acid. Optionally, 2,3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or more labeled proteins ofthe protein standard can be selectively labeled with a differentfluorescent dye on a fourth amino acid, such that the protein is labeledwith two different fluorescent dyes with different excitation/emissionwavelengths.

The protein standard described herein can span a molecular weight rangeof from about 1 kDa to about 500 kDa or more, from about 1 kDa to about300 kDa or more, from about 1 kDa to about 250 kDa or more, from about 1kDa to about 200 kDa or more, from about 5 kDa to about 300 kDa or more,from about 5 kDa to about 250 kDa or more, from about 8 kDa to about 260kDa or more, from about 10 kDa to about 200 kDa or more, from about 10kDa to about 150 kDa or more, from about 10 kDa to about 100 kDa ormore, from about 10 kDa or less to about 150 kDa, or from about 10 kDaor less to 300 kDa.

In some aspects, the protein standard includes 3, 4, 5, 6, 7, 8, 9, 10,11, 12, 13, 14, 15, or more labeled proteins that differ in size fromone another by an increment of about 5 kDa, 10 kDa, 15 kDa, 20 kDa, 25kDa, 30 kDa, 35 kDa, 40 kDa, 45 kDa, 50 kDa, 55 kDa. 60 kDa, 65 kDa, 70kDa, 75 kDa, 80 kDa, 85 kDa, 90 kDa, 95 kDa, 100 kDa, 105 kDa, 110 kDa,115 kDa, 120 kDa, 125 kDa, 130 kDa, 135 kDa, 140 kDa, 145 kDa, 150 kDa,155 kDa, 160 kDa, 165 kDa, 1700 kDa, 175 kDa, 180 kDa, 185 kDa, 190 kDa,195 kDa, 200 kDa, 205 kDa, 210 kDa, 215 kDa, 220 kDa, 225 kDa, 230 kDa,235 kDa, 240 kDa, 245 kDa, 250 kDa, 255 kDa. 260 kDa, 265 kDa, 270 kDa,275 kDa, 280 kDa, 285 kDa, 290 kDa, 295 kDa or more.

In some aspects, the dual functional protein ladder having catalytic andfluorescent activity (e.g., IR fluorescent activity) can be mixed with apre-stained protein ladder. The dual functional protein ladder and thepre-stained ladder can contain one or more purified proteins of the samemolecular weight. Alternatively, the dual functional protein ladder andthe pre-stained ladder can contain one or more purified proteins ofdifferent molecular weights. In some aspects, the purified proteins ofthe dual functional protein ladder and the pre-stained protein ladderare the same molecular weights and/or have the same mass.

In other aspects, the dual functional protein ladder having catalyticactivity and pre-stained proteins can be mixed with a IR fluorescentprotein ladder. The dual functional protein ladder and the IRfluorescent protein ladder can contain one or more purified proteins ofthe same molecular weight. Alternatively, the dual functional proteinladder and the IR fluorescent protein ladder can contain one or morepurified proteins of different molecular weights. In some aspects, thepurified proteins of the dual functional protein ladder and the IRfluorescent protein ladder are the same molecular weights and/or havethe same mass.

The purified proteins of the protein standard can be purified fromnaturally occurring sources. Alternatively, the proteins can beexpressed recombinantly according to standard methods recognized bythose skilled in the art. The proteins can also be chemicallysynthesized according to standard methods.

The multifunctional protein molecular weight ladder described herein hassustainable and long-lived peroxidase-like activity. Such a proteinladder is versatile and suitable for using to estimate (e.g., determine)molecular weights in multiplex protein detection assays that measurechemiluminescence and fluorescence simultaneously or sequentially.

C. Methods of Labeling Pre-Stained Molecular Weight Protein LaddersUsing For Example, a Mono MP-11 NHS Ester

Described herein is a method where a mixture of proteins, e.g., proteinmarkers, standards, or a protein ladder, can be conjugated to moietieswhich contain visible color, catalytic activity, fluorescent activity,and any combination thereof. The conjugation process uses the novel ideaof mean molecular weight (“mean MW”) of the proteins in the mixture toachieve consistent and reproducible conjugation across the molecularweight range of the proteins in the mix. For example, in a mixture of 8proteins of molecular weight 250 kDa, 130 kDa, 80 kDa, 50 kDa, 37 kDa,25 kDa, 15 kDa, and 10 kDa, the mean molecular weight of the mixture is74.6 kDa. The mean MW is used to calculate the estimated molarconcentration of the protein mixture. Depending on the moiety(s) to beconjugated, 1-40 molar equivalents of material are added to thelabeling/conjugation reaction simultaneously or step wise (e.g.sequentially). In some aspects, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30,31, 32, 33, 34, 35, 36, 37, 38, 39 or 40 molar equivalents of MP-11 orother microperoxidase, the visible dye or the fluorescent dye is used inthe reaction.

The optimal number of equivalents is determined by the ultimatefunctionality of the moiety in the mixture. The number of molarequivalents used in the labeling reaction may be determined empirically.For example, robust chemiluminescent activity requires labeling using 30molar equivalents of catalyst to provide a chemiluminescent signal froma variety of substrates using film or digital imagers such as Odyssey Fcor C-Digit imager. For fluorescent dyes, optimal fluorescence signal canbe achieved by using e.g. 4-8 molar equivalents of dye. For visibledyes, optimal color or visible signal can be achieved using e.g. 8-16equivalents of dye. The optimal number or molar equivalents depends onthe desired functionality of the moiety in the protein mixture.

Up to three moieties or functionalities, e.g., 1, 2 or 3 functionalitiescan be conjugated to a portion of proteins in a mixture. The firstconjugation may label a portion of proteins with one or more visibledyes. A second functionality may be the addition of a fluorescent dye. Athird functionality may be the addition of a catalytic moiety whichfunctions as an enzyme analog.

In one embodiment, a visible dye label is applied first to theprotein(s). The second labeling can be to a fluorescent dye where thenumber of molecules to be conjugated to each protein is relativelysmall. The third functionality to be added can be a catalytic moiety(e.g., MP-11) which introduces a relatively large number of molecules toprovide chemiluminescent functionality. An example of this would beMP-11 NHS ester labeling. After the addition of any moiety which doesnot utilize NHS ester chemistry, e.g., visible dye, the order of themoiety addition should follow from least to most molar equivalentsneeded to provide the desired functionality.

When the conjugation between the moiety and protein mixture utilizes areactive NHS ester to covalently attach the moiety to the proteins, thelabeling is performed in an amine-free buffer such as 50 mM sodiumphosphate, pH 7.7-9.0 or 50 mM sodium phosphate, pH 7.7-9.0 for two-fourhours at ambient temperature. Optimization of the labeling parameterscan be required. First, the optimum number of molar equivalents of eachmoiety based on the average molecular weight of the protein mix whenadded individually to the labeling reaction is determined. Next, theprotein mixture is prepared in a buffer that is amenable to theconjugation chemistry. Then, the moiety is added to the mixture andincubated to allow the covalent attachment of the moiety to the proteinsin the mix. The final step includes the removal any unreacted moietiesusing commonly used protein purification techniques, such as sizeexclusion chromatography or dialysis with appropriate molecular weightcutoff limits.

Alternatively, the conjugation can be performed in a one pot (onereaction vessel) reaction if the labeling conditions for each moiety aresimilar. As described above, the initial step includes determining thenumber of molar equivalents of each moiety to be added by optimizingindividual labeling of the protein mix. Next, a cocktail of the moietiesin pre-determined ratios is prepared. Finally, the moiety(s) are mixedwith the protein mixture, incubated to promote covalent attachment, andpurified to eliminate unreacted moieties.

After labeling a protein ladder with a monoperoxidase and optionally, atleast one other chemical moiety, the labeled protein ladder may beseparated from any side reaction products and any free hydrolyzedproduct resulting from normal hydrolysis. In some aspects, sizeexclusion chromatography (e.g., gel filtration chromatography) using acolumn of dextran, polyacrylamide, dextran-polyacrylamide, agarose, orthe like, and/or dialysis may be used to purify the labeled ladder. Agel filtration media can be selected with a suitable molecular weightcut-off according to the molecular weights of the labeled proteins.

In some aspects, MP-11 labeled protein ladder is purified with a sizeexclusion column, e.g., spin column. In some instances, the purifiedlabeled protein ladder is diluted in a buffer containing Tris, glyceroland SDS. In some aspects, the buffer, such as a storage buffer, includes62.5 mM Tris-H₃PO₄ (pH 7.5), 30% (v/v) glycerol and 2% SDS.

The labeled protein ladder can be stored in a buffer, e.g., storagebuffer. The selected storage buffer can, for example, stabilize theprotein ladder, stabilize three functionalities of the protein ladder(visible dyes, fluorescent dyes and chemiluminescence), preventdenaturation, inhibit degradation, or preserve the protein ladder forlong-term storage (e.g., up to one year) at various temperaturesincluding room temperature, 4° C., −20° C., and −80° C. The storagebuffer can include 50 mM-65 mM Tris, pH 6.8-7.5, 20-40% (v/v) glyceroland 1-3% SDS. In some aspects, the storage buffer contains 62.5 mMTris-H₃PO₄ at pH 7.5, 25% (v/v) glycerol and 5% SDS. In preferredaspects, EDTA, DTT and NaN₃ are not present in the storage buffer. Theinventors have discovered that storing MP-11 labeled proteins in thepresence of DTT, EDTA, NaN₃ or any combination thereof reduces (e.g.,decreases) the catalytic-like activity of the microperoxidase.

Provided herein is a kit for generating a catalytic protein molecularweight ladder that includes a plurality of purified proteins ofdifferent molecular weights (i.e., a protein ladder), monomicroperoxidase NHS ester (e.g., mono acetylated MP-11 NHS ester), and alabeling buffer. In some aspects, the labeling buffer is an amine-freebuffer, e.g., a buffer that is free or substantially free of amine. Insome instances, the labeling buffer is 50 mM sodium phosphate at aboutpH 7.7-9.0. In some aspects, the kit also contains storage buffer thatis free or substantially free of one or more of the following: DTT,EDTA, NaN₃, and antibodies with catalytic activity. In other aspects,the kit includes an instruction manual. In certain instances, the bufferoptionally comprises 0.1%-1% SDS.

D. Methods of Using Multifunctional Protein Molecular Weight Ladders forQuantitating the Amount of Protein in a Test Sample

For detecting multiple proteins in western blotting, it is desirable touse a protein ladder that shows up on digital imaging equipment and alsocorresponds to signal emitting from target detection. Themultifunctional protein molecular weight ladder provided herein isapplicable for estimation of at least three molecular weights of targetproteins simultaneously or subsequently. Chemiluminescent andfluorescent protein bands at 700 nm and 800 nm can be visualized withinstruments equipped with dual-mode detection systems, such as theOdyssey® Fc for NIR fluorescence and chemiluminescence, individualsystems, such as the Odyssey® CLx, Odyssey® Sa, CDiGit® blot scanner,and other chemiluminescence imaging systems.

In yet another embodiment, the present invention provides a method forperforming a Western blot analysis, the method comprising:

-   -   (a) loading a protein standard comprising a plurality of        purified proteins of differing molecular weight, wherein at        least a first portion of the purified proteins is covalently        labeled with a mono microperoxidase (MP) N-hydroxy succinimidyl        (NETS) ester;    -   (b) electrophoresing the plurality of proteins to form a        separated protein mixture;    -   (c) transferring the separated protein mixture to a membrane;    -   (d) adding a chemiluminescent substrate to form a        chemiluminescent signal; and    -   (e) capturing the chemi-signal with a film or an imager.

In some aspects, the membrane is nitrocellulose or PVDF. In someaspects, an unknown protein is loaded with the protein standard.

In some instances, the multifunctional protein ladder is run on aSDS-PAGE polyacrylamide gel, e.g., NuPAGE® Bis-Tris gel with 1× NuPAGE®MES-SDS Running Buffer (Thermo Fisher Scientific, Waltham, Mass.) at160V, constant voltage for about 55 minutes. The pre-stained proteins ofthe ladder can be visualized and imaged. The fluorescently labeledproteins can be analyzed using a digital imaging system equipped withfluorescence detectors. Alternatively, the SDS-PAGE gel containing theresolved multifunctional protein ladder can undergo western blottingprocessing. The proteins can be transferred to a membrane, e.g., anitrocellulose, PVDF or the like membrane).

For western blotting, the MP-11 labeled proteins of the protein laddermay be detected by reacting the microperoxidase with a suitablesubstrate, including, but not limited to, a chromogenic substrate (e.g.,3,5-di-t-butyl-catechol (DTBC), 3,3′,5,5′-tetramethylbenzidine (TMB),2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS),4-aminoantipyrene (4AAP), 4-chloro-1-naphthol (4CN),5-bromo-4-chloro-3-indolyl phosphate/nitroblue etrasolium (BCIP/NBT), ordiaminobenzidine (DAB)), a chemiluminescent substrate (e.g., luminol,isomers thereof, or derivatives thereof) and a fluorescent substrate(e.g., 4-(N-methylhydrazino)-7-nitro-2,1,3-benzooxadiazole (MNBDH)). Insome aspects, a chemiluminescent signal is generated after applying aluminol-based substrate to the MP-11 labeled protein ladder of a westernblot. The signal can be detected by digital imaging equipment such asthe Odyssey® Fc or similar devices. Optionally, the fluorescence labeledproteins of the protein ladder may be detected using a digitalfluorescence detector.

IV. Examples

The following examples are offered to illustrate, but not to limit, theclaimed invention.

Example 1. Synthesis of Mono MP-11 NHS Esters

This example illustrates a two-step method for synthesizing mono MP-11NHS esters (FIG. 1). The inventors have discovered that blocking thefirst primary amino moiety in MP-11 using sulfo-NHS acetate, in aqueoussolution (pH 8.5 phosphate buffer), and at low MP-11 concentrationimproves conjugation efficiency and eliminates MP-11 aggregation. TheN-acetylated MP-11 was purified in the second step of the reaction.Purification of N-acetylated MP-11 (pH=2-5) did not inhibitperoxidase-like activity of the modified MP-11. The synthesis of MP-11NHS ester is performed in organic solvent. However, the unmodified MP-11is not soluble in such solvents. N-acetylating MP-11 increases thesolubility of MP-11, and thus, the mono MP-11 NHS ester can be preparedin organic solvent by using a homobifunctional NHS ester. Mono MP-11 NHSester retained peroxidase-like activity, resulting in less aggregation,high purity and better yield.

Example 2. Methods for Labeling Protein Ladders Using Mono MP-11 NHSEsters

The protein molecular markers, such as unstained and pre-stained proteinladders were labeled with mono MP-11 NHS ester as described above. Theprotein ladders included those containing one or more molecular weightproteins of about 260 kDa, 160 kDa, 140 kDa, 130 kDa, 100 kDa, 90 kDa,70 kDa, 55 kDa, 50 kDa, 40 kDa, 38 kDa, 35 kDa, 30 kDa, 25 kDa, 15 kDa,10 kDa, 8 kDa, and any combination thereof. Additionally, proteinladders labeled with a fluorescent moiety, such as a near-infrared dyethat can be visualized by SDS-PAGE or western blotting using afluorescent detection instrument, were labeled with mono MP-11 ester.Total protein concentration of the protein ladder was estimated in therange of 0.25 mg/ml to 1 mg/ml. The mean molecular weight of proteinladder was estimated. Protein concentration, reaction time, molar ratiobetween mono MP-11 NHS ester and proteins was optimized for the labelingreaction.

The labeled protein markers were resolved by standard gelelectrophoresis. NIR fluorescence was detected using the Odyssey®infrared imaging system (LI-COR). The protein ladder was alsotransferred from the gel to a blotting membrane using standard westernblotting methods. A chemiluminescent substrate was applied to the blotand digitally imaged with a CCD camera system such as the Odyssey® Fc(LI-COR).

The protein markers all displayed pre-stained properties,peroxidase-like activity and fluorescent properties. As shown in theFIGS. 2A-C, the multifunctional markers show discrete chemiluminescentprotein bands upon developing with luminol-based chemiluminescencesubstrates.

Additives, such as EDTA, DTT and NaN₃ are commonly included in storagebuffers for molecular weight markers because they can increase long termstability. However, EDTA, DTT and NaN₃ can complex with iron in the hemegroup and may affect the catalytic activity of MP-11. NaN₃ and DTT mayalso destroy intermediates of super oxides radicals in thechemiluminescence reaction. To test the stability of the multifunctionalprotein ladder in the presence of standard additives, the ladders werestored in various buffers including those containing EDTA, DTT and NaN₃.The storage buffer was composed of 62.5 mM Tris-H₃PO₄ (pH 7.5), 2% SDSand 25% glycerol. The peroxidase-like activity was decreased when theprotein markers were stored in a buffer containing DTT, NaN₃ and/orEDTA. As shown in FIG. 3A-B, the peroxidase-like activity disappearedafter two days at room temperature (about 20-25° C.) in storage buffercontaining DTT, NaN₃ and EDTA. However, the protein ladders stored inDTT-, NaN₃- and EDTA-free buffer retained its peroxidase-like activity.

The stability of the multifunctional ladder stored in DTT-, NaN₃- andEDTA-free buffer was evaluated at different storage temperatures. FIGS.4A-B show that the protein ladders (a pre-stained protein ladder in FIG.4A; a dual function protein ladder with pre-stained proteins and IR dyecoupled proteins in FIG. 4B) were stable after 5-6 months when stored at−80° C., −20° C. and 4° C.

Example 3. Methods for Conjugating Moieties to a Mixture of Proteins

This example illustrates a method for making a multifunctional proteinladder that includes proteins labeled with IRDye® 800CW (Cat. No.830-08038, LI-COR, Lincoln, Nebr.). The method comprises the steps of:(1) dialyzing the pre-stained protein molecular weight marker in sodiumphosphate buffer, (2) determining the number of molar equivalents of theIRDye® 800CW to be added based on the mean molecular weight of theprotein ladder, (3) performing the labeling reaction, and (4) removingunreacted IRDye® 800CW after the reaction is complete.

The protein molecular weight marker was dialyzed in a hydrated 2000 MWCOdialysis cassette in 50 mM sodium phosphate buffer, pH 8.5 for overnight(or at least 16 hours) at ambient temperature.

The amount (mass) of the protein molecular weight marker in mg wasdetermined by multiplying the concentration supplied by the manufacturerby the volume in ml of the marker to be labeled. To calculate the amountof IRDye® needed to label the dialyzed marker, the following equationwas used:

${\frac{{Protein}\mspace{14mu} {Marker}\mspace{14mu} {Mass}\mspace{14mu} ({mg})}{{Average}\mspace{14mu} {MW}\mspace{14mu} {Protein}\mspace{14mu} {Marker}} \times {Dye}\mspace{14mu} {Molar}\mspace{14mu} {Euivalents} \times {Dye}\mspace{14mu} {MW}} = {{Dye}\mspace{14mu} {Needed}\mspace{14mu} ({mg})}$

-   -   MW=Molecular Weight (g/mole)

The appropriate volume of dye needed for the labeling reaction was addedto the dialyzed protein marker and vortexed gently but thoroughly. Thelabeling reaction was incubated at about 20° C. for 2-4 hours andprotected from light. Optionally, the reaction is then stored overnight(12-24 hours) at 4° C. protected from light to allow the reaction to goto completion and the residual dye to hydrolyze. Unreacted dye wasremoved by size exclusion chromatography using a Zeba Desalt Spin Column(Pierce) with a 7K MWCO.

Example 4. A Pre-stained Chemiluminescent Protein Ladder

A pre-stained chemiluminescent protein ladder was designed to provide aladder of convenient and consistent protein sizes (8-250 kDa) for usewith polyacrylamide gels and on Western membranes where chemiluminescentdetection is used. The Ladder offers both pre-stained andchemiluminescent functionalities. The Ladder is suitable for use withfilm and a variety of chemiluminescent substrates and can be detected onOdyssey® Fc, CDiGit® Blot Scanner, and other imaging modalities capableof chemiluminescent detection. In gels, the Ladder can be used tovisualize progress of the protein separation during electrophoresis andto determine the molecular weight of unknown proteins based on theirrelative mobility. In blotting applications, the Ladder can be used tomonitor protein transfer and as a reference to determine the molecularweight of proteins of interest.

FIG. 5 shows representative visible (left) and chemiluminescent (right)images of the chemiluminescent protein Ladder resolved on a 4-12%bis-Tris gel and transferred to a nitrocellulose membrane via wet tanktransfer. The membrane was exposed to WesternSure ECL Substrate (LI-COR,P/N 926-80100) and scanned on a C-DiGit Blot Scanner for 12 minutes.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, one of skill in the art will appreciate that certainchanges and modifications may be practiced within the scope of theappended claims. In addition, each reference provided herein isincorporated by reference in its entirety to the same extent as if eachreference was individually incorporated by reference.

What is claimed is:
 1. A compound of formula I:

wherein R¹, R², R³, and R⁴ are each members independently selected fromthe group consisting of hydrogen, an amine protecting group and L,wherein L is a linking group.
 2. The compound of claim 1, wherein R¹ andR² are each independently a member selected from the group consisting ofhydrogen, an acetyl (Ac), a benzoyl (Bz), a benzyl (Bn) group,tert-butyloxycarbonyl (BOC) group, a carbamate group, a carbobenzyloxy(Cbz) group, 3,4-dimethoxybenzyl (DMPM), 9-fluorenylmethyloxycarbonyl(FMOC) group, p-methoxybenzyl carbonyl (Moz) group, p-methoxybenzyl(PMB), p-methoxyphenyl (PMP) group, or Tosyl (Ts) group.
 3. The compoundof claim 1, wherein R¹ is acetyl and R² is hydrogen.
 4. The compound ofclaim 1, wherein R⁴ comprises an NHS ester.
 5. The compound of claim 1,wherein the compound has the formula:


6. The compound of claim 1, wherein R⁴ is L.
 7. The compound of claim 6,wherein L is L^(c), and L^(c) is a member selected from the groupconsisting of an amide, a thioether, and an alkylamine.
 8. A kit forpreparing the protein standard, the kit comprising a plurality ofpurified proteins of differing molecular weight, a compound of FormulaI, and a labeling buffer, wherein the compound of Formula I has theformula:

wherein R¹, R², R³, and R⁴ are each members independently selected fromthe group consisting of hydrogen, an amine protecting group and L,wherein L is a linking group.
 9. The kit of claim 8, wherein thelabeling buffer is an amine-free buffer.
 10. The kit of claim 8, whereinthe labeling buffer contains SDS.
 11. The kit of claim 8, furthercomprising a storage buffer.
 12. The kit of claim 8, wherein the storagebuffer is substantially free of DTT, EDTA and NaN₃.
 13. A method forpreparing a multifunctional protein standard comprising a plurality ofpurified proteins labeled with at least two chemical moieties selectedfrom the group consisting of a visible dye moiety, a catalytic moiety, afluorescent dye moiety, a chemiluminescent moiety, and a combinationthereof, said method comprising: (a) determining the optimum number ofmolar equivalents of the at least two chemical moieties based on theaverage molecular weight of the plurality of purified proteins whenlabeled individually; (b) preparing the plurality of purified proteinsin a buffer amenable to the conjugation chemistries of the at least twochemical moieties; and (c) incubating the at least two chemical moietiesand the plurality of purified proteins under conditions such that the atleast two chemical moieties covalently attach to the proteins to producethe multifunctional protein standard.
 14. The method of claim 13,further comprising purifying the multifunctional protein standard. 15.The method of claim 13, wherein purifying the multifunctional proteinstandard comprises size exclusion chromatography or dialysis.
 16. Themethod of claim 13, wherein incubating the at least two chemicalmoieties and the plurality of purified proteins comprises simultaneouslyconjugating the chemical moieties to the proteins.
 17. The method ofclaim 13, wherein incubating the at least two chemical moieties and theplurality of purified proteins comprises sequentially conjugating thechemical moieties, wherein a chemical moiety with an NHS ester isconjugated to the proteins after a chemical moiety without an NHS ester.18. The method of claim 13, wherein incubating the at least two chemicalmoieties and the plurality of purified proteins comprises sequentiallyconjugating the chemical moieties, wherein the order of conjugating isfrom the least to most molar equivalents needed for each chemicalmoiety.
 19. The method of claim 13, wherein at least two purifiedproteins of the multifunctional protein standard are labeled with acatalytic moiety and a visible dye moiety.
 20. A method for performing aWestern blot analysis, the method comprising (a) loading a proteinstandard comprising a plurality of purified proteins of differingmolecular weight, wherein at least a first portion of the purifiedproteins is covalently labeled with a mono microperoxidase (MP)N-hydroxy succinimidyl (NHS) ester; (b) electrophoresing the pluralityof proteins to form a separated protein mixture; (c) transferring theseparated protein mixture to a membrane; (d) adding a chemiluminescentsubstrate to form a chemiluminescent signal; and (e) capturing thechemi-signal with a film or an imager.